You got it! I always thought it was weird how they did that. Why not have an straight switch of styles. I get that they have leftover parts but why not just call it the previous year?
Because they hold over the heavier duty/work/fleet vehicles for a couple of years while they debug the new ones on individual customers.
It is necessary when switching body styles means Podunk, Ky, won't be able to use the $5,000 _______ that they already have on the shelf for their next police Tahoe, or the fire chief's silverado they had budgeted to procure a year before (or two years before).
Not only that, they need data on reliability and maintenance costs, before they will consider switching. They get one last chance to buy fleet vehicles that will integrate seamlessly into the existing fleet, or to replace the very oldest ones with a model that can use the same add-ons, before they have to buy all new vehicles and all new accessories. OEMs get an extra year for powertrain development and testing on the heavier duty variants this way also.
IIRC the last time GM made a clean break on 3500 and under was the 72/73 switch. And they only had single cabs, blazer, and suburbans in the 72, converting also to single cabs, ( 3+3 3500 only) new blazer, and suburban in 3 trim levels with mostly dealer install options, and only 3 power plants IIRC, i6, 350 and 454.
At least it is only one year... the 87/88 GM switch took 3 years for blazer, suburban, and HD pickups. 88, 89, and 90.
Ford took 4 years to change their Bronco and HD pickups... from 94 on the f150 to 98 for the rest...